When drilling offshore wells, the most common method is to drill with a single gradient fluid with the fluid column extending from the bottom of the well back to the drilling rig. Even in shallow water, this sometimes represents a significant challenge if the formation cannot withstand the hydrostatic pressure from the fluid column. An example would be drilling from a jack-up where hydrostatic fluid pressure causes fracture under the shoe of the conductor pipe.
As water depth increases, so do problems associated with having the fluid column all the way back to the rig. The effect of the water depth is particularly significant in the upper part of the well and, consequently, if it can be eliminated, drilling performance can be significantly improved. For this reason, the industry has been working with developing dual gradient drilling technology for decades.
Several methods can be classified as dual gradient drilling. The most common are the seabed pumping and mid-riser pumping methods. For seabed pumping, the drilling riser typically is filled with seawater and the drilling mud is pumped (or lifted) back to the rig from the seafloor. This represents a drilling method in which the effect of the water depth can be eliminated and the well basically drilled as if the drilling rig were sitting on the ocean floor. The same can be achieved with the mid-riser pumping method, but in this case, the riser is typically filled with a gas.
Another significant problem is the effect of dynamic pressure loss (friction) while circulating drilling mud through the wellbore in order to transport drill cuttings back to the rig. In deep water, the margin between pore pressure and fracture pressure is small; hence, there is a narrow drilling window. For static conditions, the mud must have a weight or density great enough to create a wellbore pressure that is above the pore pressure (static overbalance). When the rig mud pumps are started to circulate fluid around the well, the wellbore pressure increases as a function of friction and the pressure may exceed the fracture gradient (typically at the shore above) and result in lost circulation. This may again result in a kick. For low-pressure reservoirs or depleted fields, this problem is significant and may make the wells undrillable with conventional technology.
This dynamic effect, equivalent circulating density (ECD), is a significant drilling challenge in terms of safety and lost time.
The Business Incentive
The business incentive of dual gradient technologies is to improve both safety and drilling performance by eliminating the effect of water depth. In this way, intermediate liners and associated time and cost will be eliminated. Dual gradient technologies enable drilling with a close-to-constant bottomhole pressure and cause little pressure change between static and dynamic conditions.
The pump system not only controls the pressure accurately, but also detects minor changes in flow and fluid density. Well control will thereby be improved and even minor influxes or losses will be detected.
Except for riserless mud recovery top-hole technology, the industry has not been able to develop and deploy dual gradient drilling commercially. The EC-Drill system is a “first out” for post-blowout preventer drilling and is more of a managed pressure drilling (MPD) technology than a full dual gradient technology. The primary purpose of EC-Drill is to eliminate the effect of ECD and enable drilling with close to constant bottomhole pressure.
Technical Details
EC-Drill is an MPD technology that falls within the mid-riser pumping technology of the dual gradient family (Fig. 1). It uses a subsea mud pump that is connected to the conductor pipe or marine drilling riser (Fig. 2).
On a semisubmersible rig, the EC-Drill is attached to the drilling riser in the moonpool and is run together with the riser. The EC-Drill can be run in automatic mode, keeping a desired bottomhole pressure constant, or in manual mode in which the EC-Drill control system maintains a constant riser fluid level set by the driller.
The EC-Drill should not be used to drill underbalanced. A static overbalanced mud weight should be used. For a planned stop in circulation, the riser level should, if necessary, be increased to the point at which static overbalance is obtained. If the rig pumps unexpectedly stop, the annular preventer or a similar device should be closed to prevent static underbalance.
Applications
The target applications are a narrow drilling window, high ECD, and low mud weight environments in which losses are an issue.
The technology has been used nine times. The following are EC-Drill applications to date:
- One well drilled from a jack-up for BP Egypt
- One well drilled from a jack-up for BP in the Caspian Sea
- Four wells drilled from a jack-up for Petrobras in Brazil
- Three wells drilled from a semisubmersible in deep water, eastern Gulf of Mexico
Case Studies
Three wells have been drilled with EC-Drill from a semisubmersible in a deepwater/low-mud-weight environment offshore Cuba.
Well No. 1. The EC-Drill system was only used in the 12¼-in. hole section. The formation drilled through was predominantly hard-to-very-hard limestone with softer marl (carbonate also) formations embedded. This was experienced through the entire section. Particular drilling challenges included the hard drilling environment and lack of suitable bits and tools. After a failure, the system could not be used for the 8½-in. hole section and losses were experienced.
Well No. 2. The EC-Drill system was used in the 17½-in. and 12¼-in. hole sections to well total depth. This well was drilled in the same area and with the same rig immediately after Well No. 1. The formation drilled was chalk and moderately hard limestone, sometimes with marl. The main reason for using EC-Drill was the potential for weak zones with the risk of severe losses. An additional effect was improved rate of penetration when reducing riser level and increasing circulation rate. Although the formations drilled through were hard, the 17½-in. hole section was also particularly weak with regard to the planned cementing program. The EC-Drill system was used to manage the bottomhole pressure during the cement displacement and hardening process (Fig. 3).
Successes, Failures, and Lessons Learned
On Well No. 1, there was a problem with the EC-Drill equipment caused by galvanic corrosion on the control system combined with extreme currents—up to 8 knots. A superior galvanic corrosion system was installed and Well No. 2 went smoothly. A significant increase in rate of penetration was experienced when EC-Drill was used. There were no well issues or drilling challenges on Well No. 2.
After Well No. 1, changes to the control system were made for greater robustness in operation. A superior galvanic corrosion system was implemented. Changes were also made to the flexible mud return line and its position in the moonpool. Together, these resulted in the elimination of the associated issues on Well No. 2. On Well No. 2, a significant increase in rate of penetration was seen—up to 30%. No drilling issues or losses were experienced.